The present invention relates to an optical external modulation device for modulating incident light.
Fiber optic communication technology has been developed through utilization of the ultra-low loss property of optical fiber and the essential ultra-wide band property of light, and studies for longer distance and larger capacity transmission are now being made in many countries. Now that the transmission loss by optical fiber has reached a theoretical limit, studies for faster, larger capacity transmission are of particular importance.
For turning ON and OFF an optical signal at a high speed it is customary, at present, to employ a method which directly modulates a semiconductor laser. With the direct modulation system, however, since a current of the semiconductor laser which is an oscillation element is varied at a high speed, the oscillation wavelength undergoes substantial variations with time, resulting in the oscillation spectral width becoming abnormally large as compared with the spectral width in the modulation band. Accordingly, in long distance or high-speed transmission light pulses are subjected to a wavelength dispersion by the optical fiber and distorted light pulses are received, so that no excellent transmission characteristic can be obtained. As a solution to this problem there has recently been under study a method which holds the output of the semiconductor laser constant and modulates it by an external optical modulation device at a high speed.
As optical modulation devices there have been proposed an optical modulation device which utilizes a ferrodielectric substance such as LiNbO.sub.3 and a semiconductor optical modulation device which can be monolithically integrated with a single-wavelength semiconductor laser such as a DFB laser. Of the conventional devices, the latter electroabsorption type optical modulation device is regarded as the most promising which applies an electric field to the modulation waveguide and performs an intensity modulation by an electroabsorption.
As described above, the conventional electroabsorption type optical modulation device is capable of the low-voltage modulation, the high-speed operation and the narrow-spectrum operation but is defective in that these characteristics will significantly be degraded when the incident light intensity is raised up to a practical level of several mW.